Contactors, such as electro-magnetically operated contactors for example, typically have one or more pairs of contacts and a coil which is energized to close the contacts. Such contactors are designed to have a nominal working voltage for the solenoid coil, but will pull in at a somewhat lower applied voltage. They usually operate satisfactorily with an applied voltage somewhat greater than the rated voltage. If the voltage applied to the coil is reduced after the contactor has pulled in, the contacts will remain closed until the applied voltage reaches a drop-out voltage which is appreciably lower than the pull-in voltage.
Such a contactor may be used to switch the supply from a battery to a motor, for example a hydraulic pump motor, of a battery-operated vehicle, such as fork-lift truck. As the battery becomes discharged during normal use of the vehicle, the terminal voltage decreases, and when a heavy current demand is applied to the battery, for example on starting the motor under heavy load conditions, the terminal voltage can fall to a very low value.
Since the contactor coil supply is also taken from the battery, this low terminal voltage on heavy load may be insufficient to hold in the contactor. Hence, when the load is applied, the contactor will begin to drop out, but, as the contacts open, the load will be removed from the battery and the terminal voltage will immediately rise. This will cause the contactor to pull in again, once again applying the load to the battery. This action will be repeated continuously. If the contacts continue to open and close on the high direct current load in this manner, either of two extremely serious situations can arise. The contacts may weld together, in which case the motor will become uncontrollable. On the other hand, if they continue to open and close cyclically, the contacts will burn away due to the arcing which takes place, and destruction of the contactor can result.
It is desirable to avoid this cyclic opening and closing of the contacts of an electro-magnetic contactor.